Super paramagnetic fluids have been known for sometime. Typically, these fluids contain finely divided magnetic particles which ar dispersed in oil by means of one or more dispersants. The oil or more generically, the carrier fluid maybe of varying types, as is the dispersement for the magnetic particles. Typically, the carrier fluid is colloidal oil.
Super paramagnetic fluids are introduced into and retained in spaces without mechanical restraint when a magnetic field is employed. This advantageous property has resulted in the use of super paramagnetic fluids as fluid seals having very low torque resistance. These seals do not generate particles during dynamic operation, in contrast to conventional lip seals. These fluid seals using super paramagnetic fluids have found extensive use as exclusion seals in disc devices for computers and as pressure seals in devices having a plurality of seals or stages. Super paramagnetic fluids are also used as heat transfer fluids between voice coils and magnetics in loud speakers. Examples of super magnetic fluids and the various compositions are described in U.S. Pat. Nos. 3,700,595; 3,764,540; 3,843,540; 3,917,538; 4,208,294; 4,285,801; 4,315,827; 4,333,988; and 4,701,276.
These magnetic fluids, particularly those described in the above referenced patents, are generally not electrically conductive. However, it is sometimes desirable that the fluids have a certain amount of conductivity. This is particularly important when the magnetic fluid is used as a seal. In many instances, it is highly desirable that static electricity be dissipated or removed from the apparatus being sealed. One method which has been provided to remove static electricity is the use of a knob which slides against the rotating shaft. Alternatively, the magnetic fluid may be made electrically conductive by addition of carbon particles, particularly in the form of graphite or by the use of an excess of cationic dispersant. These methods are described in European patent applications 208,391 and 206,516.
It is clear that there is a serious drawback in the use of an electrically conductive knob. Specifically, the knob causes friction, noise, and wear. Such knobs also require that there be more parts to assemble, thereby resulting in a complex and expensive seal.
The other alternative, the use of graphite in the magnetic colloid, may be given the desired electrical conductivity. However, it simultaneously increases the viscosity and adversely impacts on the physical properties of the colloid in other ways. An excess amount of dispersement also results in an increase of viscosity. Simultaneously, it reduces the surface tension and, if it is either acidic or alkaline, result in corrosion of the materials surrounding the solution.